The HELLAS2XMM survey

People involved at OAB: Ciliegi, Civano, Comastri, Mignoli, Pozzetti, Zamorani.

This research is in collaboration with C. Vignali (Astronomy Dept., Univ. of Bologna), M. Brusa (MPE-Garching).

The HELLAS2XMM survey is a large national project carried out in collaboration with several Italian institutes (INAF-Arcetri and Rome Observatory, INAF-IASF Milano and Univ. of Rome 3). The scientific aim of this project is the evaluation of an accurate luminosity function over a wide range of redshifts and luminosities for a large sample of hard X-ray selected sources detected in public XMM-Newton observations, covering an area of about 2 square degrees at relatively shallow hard X-ray fluxes ( $F_{2-10\, \rm keV} > 10^{-14}$ erg cm$^{-2}$ s$^{-1}$). The survey strategy has been designed to be complementary to deep pencil beam surveys by sampling a different portion of the luminosity-redshift plane in order to fill the gap between the local and the deep surveys and to obtain a complete measure of the density and evolution of X-ray selected AGN.

At present, the HELLAS2XMM survey covers 1.4 square degrees (i.e., 10 XMM-Newton fields), with a total of $\approx 230$ sources detected in the hard $2-10$ keV band and a fraction of spectroscopically identified sources of $\approx 70$%; this fraction is comparable if not even higher than that in ultra-deep X-ray surveys such as the Chandra deep fields.

The most important results from the HELLAS2XMM project in 2005 can be summarized as follows:

• the X-ray spectral analysis of 81 X-ray sources has been carried out. One-third of the sample requires absorbing column densities above $10^{22}$ cm$^{-2}$; three of these sources, with intrinsic $2-10$ keV luminosities above $10^{44}$ erg s$^{-1}$, have X-ray properties consistent with those expected for Type 2 quasars. Combining the X-ray spectral results obtained for the 81 sources mentioned above with the sample of 106 sources from the HELLAS2XMM First Degree sample, we were able to measure the column density distribution for a large sample of AGN at relatively bright fluxes.
• we were awarded Spitzer IRAC and MIPS observing time, for a total of $\approx$ 42 hours (PI: A. Comastri) to obtain near and mid-IR photometry for a selected subsample of 19 optically and X-ray obscured elusive AGN. The analysis of the data clearly suggests that the X-ray sources characterized by extreme $R-K$ colours ($>5$) are dominated by the AGN host galaxy in the $K$ band and in some of the IRAC bands, with the AGN contribution strongly increasing toward the MIPS band at 24 micron. Using different empirical and theoretical spectral energy distributions (SEDs), our goal is to estimate the AGN contribution in order to derive the broad-band SEDs of these obscured AGN and, as an ultimate scientific objective, their bolometric luminosities, masses and accretion rates of their super-massive black holes.
• One of the most intriguing findings of the HELLAS2XMM survey is the detection of a few sources characterized by X-ray luminosities typical of AGN (in the range $10^{42}$-$10^{43}$ erg s$^{-1}$), without any obvious signature of nuclear activity in their optical spectra. These sources have been named XBONGs, X-ray Bright Optically Normal Galaxies. The X-ray luminosities, the X-ray-to-optical flux ratio⁴ ranging between that of truly normal galaxies ($X/O \sim -2$) and that of AGN ($X/O \sim 0$) and their hard X-ray spectra, as inferred from the X-ray colours, all suggest that some kind of activity is taking place in their nuclei. These XBONGs, found in both ultra-deep and shallow X-ray surveys, may be AGN whose emission lines are either diluted by the host galaxy starlight, or heavily obscured, or not efficiently produced. In order to search for weak AGN signatures, we have pursued an approach based on deep near-infrared (NIR) imaging of relatively bright XBONGS. We applied the surface brightness decomposition technique to $J$ and $K_s$ good-quality images of a small sample of four XBONGS detected in the HELLAS2XMM survey using the fitting program GALFIT. The contribution of the active nucleus has been deconvolved from that of the host galaxy for two sources, and the presence of nuclear activity have been weakly detected in one complex interacting double source. Extended emission both in the X-ray and in the near-infrared has been revealed in the last source. The two-dimensional analysis indicates that the lack of the emission lines is fully justified if moderate extinction ( $A_V \sim 0.5-0.8$) is present in these objects. We were recently awarded VIMOS-Integral Field Unit (IFU) spectroscopy to shed further light on the nature of these AGNs and test the hypothesis of strong dilution from the host galaxy in more detail.
• A detailed calculation of the X-ray luminosity function of a large sample of AGN (including both shallow and deep X-ray survey data) has been finalized in 2005. Thanks to a large sample of about 500 hard X-ray selected AGN it was possible to confirm that X-ray absorption is anticorrelated with X-ray luminosity and, for the first time, to uncover a trend of increasing obscuration towards high redshifts.